Antipsychotic drugs are used for the treatment of not only schizophrenia but also problematic behavior (such as aggressive behavior, mental excitation, poriomania and delirium) caused by cerebrovascular accidents and senile dementia. However, the conventional antipsychotic drugs, dopamine D.sub.2 receptor antagonists, have a serious problem in that they cause strong extrapyramidal disorders as their side effects.
On the other hand, the structure and properties of recently discovered dopamine D.sub.4 receptor are close to those of dopamine D.sub.2 receptor, and a great difference between them is their intracerebral distribution. Regarding the intracerebral distribution of dopamine D.sub.4 receptor, it is distributed in a high concentration in the cerebral cortex frontal lobe which is related to the onset of schizophrenia, but in a small amount in the striate body which is concerned in the expression of extrapyramidal disorders. In consequence, dopamine D.sub.4 receptor antagonists have a great possibility to be used as novel schizophrenia treating drugs which do not accompany extrapyramidal disorders as the side effects that are common in dopamine D.sub.2 receptor antagonists (Nature, 350, 610-614 (1991); Nature, 358, 109 (1992); Nature, 365, 393 (1993); Nature, 365, 441-445 (1993)).
Clozapine is one of such compounds. It has been reported that affinity of clozapine for dopamine D.sub.4 receptor is higher than its affinity for dopamine D.sub.2 receptor (Nature, 350, 610-614 (1991)). It has been reported also that, unlike the case of dopamine D.sub.2 receptor antagonists, clozapine is effective in drug-resistant schizophrenia and negative symptoms and causes less extrapyramidal disorders, according to its clinical investigation (Arch. Gen. Psych., 45, 789-796 (1988)). However, clozapine has a serious disadvantage in that it generates a hemotoxin as agranulocytosis which resulted in certain mortal cases (Summary and Clinical Data, Sandoz, Canada Inc. (1990)).
In consequence, such a dopamine D.sub.4 receptor antagonist having no side effects is highly useful as a drug for the treatment of diseases such as schizophrenia, having an extremely low possibility of causing extrapyramidal disorders.